Conventional methods of producing cast coated high-gloss paper for printing include a wet casting method adapted to finish a glazed coated layer by pressing the wet state coated layer comprising a mineral pigment and an adhesive on the heated highly polished finishing surface (disclosed, for example, in Japanese Patent Publications Nos. 3407/1953 and 25160/1963), a rewet casting method adapted to once dry a coated layer of wet state, to then plasticize the layer with a rewetting liquid and to press the layer on a heated finishing surface (disclosed, for example, in U.S. Pat. No. 2,759,847, Japanese Patent Publication No. 38,005/1973 and Japanese Patent Laid-open Publication No. 102,111/1976), and a gel-casting method adapted to finish a glazed coated layer by pressing the gel state coated layer on the heated finishing surface (disclosed, for example, in Japanese Patent Publications Nos. 15,751/1963, 7,207/1965, U.S. Pat. No. 3,377,192, Japanese Patent Laid-open Publications Nos. 40,410/1976 and 51,896/1982).
In any of these conventional casting methods, coating composition containing as main components, a mineral pigment and an adhesive is applied to a paper web, the wet and plasticized coating layer on paper web is pressed against the heated highly polished finishing surface of the drum by means of a press roll at a pressure higher than the vapor pressure of water, dried, and released from the drum. However, in the wet casting method, the coated layer is frequently damaged due to abrupt evaporation of water when the paper is pressed against the surface of the drum at a temperature of higher than 90.degree. C. As a result, if the pressing pressure is made considerably higher than the vapor pressure in order to prevent the coated layer from being damaged, it is impossible to maintain a sufficient amount of coating composition on the paper web, and a breakage or crack of paper sometimes occurs. Thus, in the conventional wet casting method, the temperature of the drum cannot be raised higher than 90.degree. C., and the operation has to be made at a low speed.
U.S. Patent No. 2,316,202 discloses a wet casting method using a backing roll, in which method a water pool is retained between a pressing roll having a plane(smooth) surface and the backing roll, thereby cleaning and removing coating composition adhered to the pressing roll. However, since this method prevents the transfer of water to the paper by the nip pressure of the pressing and backing rolls, the paper cannot be wetted. In Canadian Patent No. 888,099, a pressing roll is brought into contact with the wet surface of a roll to clean the surface of the pressing roll, and the back surface of the paper is wetted by the water adhered to the pressing roll, so as to ensure the intimate contact of the paper with the surface of the drum. However, since the surface of the pressing roll is plane in this art, the back surface of the paper cannot be wetted to such a degree as intended by this invention.
On the other hand, in conventional rewet casting method and gel-casting method, the coated layer before being pressed against the highly polished surface of the drum is once dried and gelled. Thus, the coated layer is not damaged as in the wet casting method even if the layer is pressed at a high pressure by the drum heated at a temperature higher than 90.degree. C. Therefore, a cast coated paper of high quality can be produced at a high speed. However, the cast coated paper thus obtained by the high temperature and high pressure casting method forms, after being released from the surface of the drum, a curl such that the paper is curled with the cross direction of the paper as the axis of the curl, the high-gloss cast finished surface being on the outside of the curl and the back surface (uncoated or coated surface) being on the inside of the curl (Such a curl will hereinafter be referred to as "CD curl".). The CD curl causes troubles in paper feed in a multicolor printing machine and seriously affects the efficiency of printing.
In order to obviate this CD curl, moisture control unit, a humidifier or a curl breaker is used in some methods. In any of these methods, it is necessary to wet the high gloss surface or squeeze it with a roll, the gloss and the smoothness of the cast coated paper are seriously reduced. In addition, the CD curl feasibly occurs as the temperature of the drum is raised, and particularly the tendency of causing the CD curl is very strong in casting at a temperature higher than 100.degree. C. Even if the pressing pressure is increased, the tendency of causing the CD curl increases. In the conventional rewet casting method in which the operation at a higher temperature of the drum and at a higher pressure of the pressing roll is desired so as to obtain excellent cast coated paper, this curl may be a fatal drawback.
In view of above-described present status, the inventors of the present invention have studied to prevent the CD curl on the basis of their long experience in the production of the cast coated paper. Particularly, the inventors have paid attention to the fact that the CD curl seldom occurs on art paper and coated paper for printing, nor in a wet casting method for producing cast coated paper but it occurs particularly in the rewet casting method and the gel-casting method which are carried out at a high temperature and a high pressure . Study for the prevention of the CD curl has been made. As a result, it is concluded that such a CD curl occurs due to the abrupt moisture content gradient in the thickness direction of base paper produced when the wet coated layer is pressed against the surface of the heated drum at a high temperature and a high pressure. In ordinary art paper and coated paper, moisture within the coated layer evaporates from both sides of the paper, and therefore extreme moisture content gradient does not occur. In the wet casting method in which moisture within the coated layer evaporates through the base paper layer from the back surface, the temperature of the drum and the pressure of the pressing roll are low and the coated layer containes much moisture, thus abrupt moisture content gradient not ocurring in the thickness direction of the base paper. In the rewet casting method and gel casting method, the moisture is abruptly evaporated by high temperature high pressure and shifted to the back surface of the base paper when the coated layer is pressed against the surface of the heated drum. Therefore, an abrupt moisture content gradient occurs in the thickness direction of the base paper when the coated layer is pressed, more moisture being contained in the coated layer of the base paper, less moisture being contained in the back surface thereof. As a result, fiber in the coated layer, i.e. the surface to be finished by casting is stretched as compared with fiber in the back surface. In addition, since the coated layer is squeezed by the high pressure of the pressing roll in the state that the moisture content gradient takes place in this manner, the fiber in the finished side is more stretched and the CD curl is aggravated.
The inventors of the present invention have concluded that if the above-described moisture content gradient in the thickness direction of the base paper is eliminated it will be possible to prevent the CD curl which has been considered to be the greatest drawback in the high temperature and high pressure casting method such as a conventional rewet casting method. The inventors have successfully eliminated the above-described moisture content gradient by wetting the back surface of the paper at or before the pressing nip between the pressing roll and the drum.